1. Field of the Invention
The present invention relates to oriented polyethylene polymer composites and a solid state drawing process for preparing oriented polyethylene polymer composites.
2. Description of Related Art
Oriented polymer composites (OPCs) are of increasing interest as high strength construction materials. For example, OPCs have entered the market place in the form of decking and fencing materials. Polymer materials offer an advantage over traditional materials such as wood due to the longevity and low maintenance of the polymer materials. Orientation of polymer compositions introduces strength in the polymer materials that rivals that of wood. Moreover, recent developments in obtaining cavitation void volume in large dimension OPCs enables production of OPCs having densities equal to or less than wood while still approaching the strength of wood. The use of solid state drawing dies has revolutionized the ability to draw large dimension OPCs having reproducible and precise shapes and is facilitating the expansion of OPCs into market avenues such as trim and other building materials that have intricate profiles.
Solid state drawing, particularly solid state die drawing can result in cavitation during drawing. Cavitation decreases the density of a drawn polymer by introducing voids in the drawn polymer. Polymer compositions containing filler have a greater tendency to cavitate than polymer composition without filler. The extent of cavitation is dependent upon the number of filler particles and to some extent the drawing rate, with an increase in either resulting in an increase in cavitation. One study has found that the extent of cavitation as a function of drawing rate reaches a plateau, or levels off, at a drawing rate of 1.2 meters (4 feet) per minute or faster (see, United States published patent application 2008-0111278). Drawing sufficiently fast so as to achieve a constant level of cavitation is desirable to ensure product density uniformity even with slight drawing rate variations.
Polypropylene has been the most popular polymer for use in producing OPCs. However, polyethylene is another low cost commodity plastic that could be used to manufacture OPCs. Polyethylene offers better inherent weatherability and toughness than polypropylene and so offers advantages as an OPC over polypropylene. Polyethylene OPCs would be cost effective, particularly if the polyethylene OPC was cavitated to reduce its weight. It is desirable to be able to include filler into the polyethylene prior to drawing to help facilitate cavitation during solid state die drawing. Moreover, it is desirable to be able to draw the polyethylene in a solid state die drawing process to benefit from the control a solid state drawing die offers in OPC production.
U.S. Pat. No. 4,938,913 ('913) discloses a ram extrusion process for orienting polyethylene. Teaching in '913 suggests that orientation can occur by a die drawing process. However, there is no teaching of how to orient the polyethylene in a solid state drawing process.
U.S. Pat. No. 5,234,652 ('652) discloses a process for continuously producing oriented very high and ultra high molecular weight polyethylene through a die drawing process. However, '652 illustrates drawing polymers at a temperature at or above their softening temperature rather than solid state drawing.
A journal article describes solid state die drawing of high molecular weight polyethylene (see, Lee and Li, DIE DRAWING TECHNOLOGY OF HIGH MOLECULAR WEIGHT POLYETHYLENE, Journal of Applied Polymer Science, Vol. 49, 15-23 (1993)). FIG. 3 of that article illustrates drawing rates up to 110 millimeters per minute.
Another journal article discloses solid state die drawing polyethylene tubes at a drawing rate of less than 80 centimeters per minute (see, Taraiya and Ward, PRODUCTION AND PROPERTIES OF BIAXIALLY ORIENTED POLYETHYLENE TUBES, Journal of Applied Polymer Science, Vol. 59, 627-638 (1996)).
It is desirable to develop a process for preparing OPCs of filled polyethylene by solid state drawing processes, especially a solid state die drawing process, in order to obtain cavitated OPCs of polyethylene. Moreover, it is desirable to prepare such OPCs at a drawing rate greater than 1.2 meters (four feet) per minute in order to benefit from stable extent of cavitation. Even faster drawing rates are desirable to increase production rates.